The following publications are believed to represent the current state of the art:
U.S. Pat. Nos. 5,699,821; 6,068,011; 6,039,302; 6,706,022; 6,745,998; 6,964,406; 7,044,441; 7,100,890; 7,104,520; 7,140,592; 7,182,313; 7,306,198; 7,497,848; 7,530,546 and 7,559,530.
U.S. Patent Publication Nos. 2007/0088324; 2007/10088324; 2008/183155 and 2009/0177170.